1. Field of the Invention
The invention pertains to the structure, fabrication of a microfluidic device and methods for conducting analysis in microfluidic devices.
2. Background of the Art Related to the Invention
Analysis utilizing specific binding pairs such as antigen/antibody; complementary DNA/DNA; DNA/RNA; RNA/RNA; biotin/avidin containing pairs are widely known in the art. Techniques for manufacturing and utilizing microfluidic devices are also well known. The art also discloses various techniques for DNA sequencing based on complementary binding of DNA.
DNA probe array technology, which utilizes binding of target single stranded DNA onto immobilized DNA probes has wide applications. A large amount of research and development activities have been carried out with different technology emphasis. For example, same technologies are focused on probe placement by mechanical means. Other technologies are focused on in-situ probe synthesis that is advantageous in producing large arrays. Additionally, other technologies are focused on gel pad arrays using photopolymerizaion and piezoelectric liquid dispensing technologies.
A common challenge to all DNA hybridization technologies is the lack of control of stringency for each individual probe site. The DNA hybridization process occurs at specific temperature and salinity conditions and varies with DNA sequences. For DNA probe arrays, since the DNA probe sequences are different, hybridization recognition is never perfect under a uniform stringency condition for the entire probe array. The problem is most obvious for short duplexes which often results in single base mismatches. One can minimize the effect of mismatched hybridization by using large probe site redundancy. Stringency control has been provided for each probe site by controlling the electrophoretic movement of oligonucleotides. To successfully implement this later scheme, a meticulously engineered permeation layer is required to prevent DNA molecules or labeling agents being damaged by direct electrolysis or by the product of the electolysis.
In addition, the current DNA array technologies have failed to provide an effective solution to maximize hybridization efficiency. For diagnostic assays, the target DNA molecules are often of minute quantities. The detection limit of the assay is determined by the sensitivity of the detection device, and also by the amount of target oligos bound to the probes during the course of hybridization. In a stationary hybridization chamber where active mixing is absent, the probability that a given target molecule hybridizes to its complementary strand on the surface is determined by diffusion rate and statistics. It takes up to tens of hours for hybridization to complete at low target concentration levels. To better utilize the target molecules and enhance the hybridization, flow through technology has been proposed where the probe arrays are placed perpendicular to the fluidic flow direction. Even with flow through technology, only a portion of the target molecules can come in contact with any specific DNA probe site.
The present invention overcomes the above technical issues by sequentially placing the DNA probe sites in microfluidic channels such that the DNA probe can efficiently contact its binding partner.
U.S. Pat. No. 5,147, 607 describes a variety of microassay devices which have microchannels in plastic materials with a reagent such as an antibody or DNA immobilized on the channel at different locations. Techniques for binding antibodies to the microchannel wall are described but techniques for binding DNA are not described. The binding of probes to the microchannel wall does not provide for optimum contact of probe and test sample. U.S. Pat. No. 5,843,767 describes microfabricated flowthrough porous apparatus for discrete detection of binding reactions such as DNA/DNA. WO/98/43739 describes porous flow channels having reagents immobilized in the chamber.